Ovenable paperboard trays are sometimes used as replacements for aluminum trays for the packaging of frozen foods. The ovenable board is often made by extrusion-coating poly(ethylene terephthalate) onto paperboard, which is formed into trays or is made into folding cartons. The trays are generally covered with lids made of poly(ethylene terephthalate)-coated board or film. Poly(ethylene terephthalate)-coated board is sometimes heat sealed to the ovenable tray, but inadequate seals are frequently obtained on trays in which food has been cooked, probably because of crystallization of the poly(ethylene terephthalate) coating during cooking. Quick-setting latex adhesives are also used, but some of these adhesives provide bonds which fail at low temperatures, and the adhesive applicators must be cleaned too frequently.
Certain four- and five-component, amorphous polyesters can be solution-coated onto poly(ethylene terephthalate) lidding film, and these coated films can be heatsealed to poly(ethylene terephthalate)-coated ovenable board but the lidded packages frequently debond when stored at low temperatures or when heated in an oven at 400.degree. F. There is a need for an improved adhesive which is coatable by extrusion or solution techniques, which will bond to both crystallized and noncrystallized poly(ethylene terephthalate) coatings on paperboard trays and adhere to both poly(ethylene terephthalate) film and poly(ethylene terephthalate)-coated paperboard lids.
Polyesters derived from terephthalic acid, ethylene glycol and diethylene glycol are well known in the art but it is believed that the critical ranges of the monomers used in making the polyester which are required for ovenable containers is novel. For example, German Pat. No. 1,110,412 discloses films of high molecular weight terephthalate polyesters based on at least one glycol of the formula HO(CH.sub.2).sub.n OH where n=2-10 and 2-5% of a polyglycol which may be diethylene glycol. U.S. Pat. No. 2,698,239 discloses photographic film supports based on a hydrophobic film coated on one surface with terephthalate polyesters derived from a polymethylene glycol containing 2 to 10 methylene groups and a compound selected from the group consisting of polyethylene glycols, saturated aliphatic dicarboxylic acids of 4 to 16 carbon atoms, their acid chlorides, or their esters. Copolyesters containing the broad range of 20-100% diethylene glycol are disclosed. This patent does not teach the use of these polyesters as adhesives. U.S. Pat. No. 4,154,918 discloses polyester adhesives based on 70-95 mol % terephthalic acid, 5-30% isophthalic acid, 20 to 60 mol % ethylene glycol, and 40 to 80 mol % diethylene glycol which have relative viscosities of 1.7-4.0 (I.V.=0.5-1.38). Up to 18 mol % of the ethylene glycol content may be replaced by a bisphenol dihydroxyalkyl ether. U.S. Pat. No. 4,156,774 discloses polyester adhesives based on 80-100 mol % terephthalic acid, 0-20 mol % of aliphatic, cycloaliphatic, or other aromatic dicarboxylic acids, 30-60 mol % ethylene glycol, 30-69 mol % diethylene glycol, and 1-10 mol % polybutylene glycol having a minimum relative viscosity of 1.4 (minimum I.V.=0.34). Up to 20 mol % of ethylene glycol or diethylene glycol can be replaced with alkylenediols having 3 to 10 carbon atoms and/or bisphenol dihydroxyalkyl esters. U.S. Pat. No. 4,048,253 discloses adhesives based on ether-type polyesters, polyurethanes, and a polyisocyanate. The ether-type polyesters are prepared by heat-treating poly(ethylene terephthalate) with glycols containing at least three carbon atoms and the ether-type polyester obtained may contain from 30 to 100 mol % diethylene glycol.